This invention relates to protective devices for communication lines and equipment and in particular to drainage reactor systems for minimizing the effect of overvoltage and current conditions caused by lightning surges or by an electric power system, either directly, such as a metallic cross of lines or a ground potential rise, or indirectly by magnetic or electric induction.
The use of mutual drainage reactors which operate only during overvoltage conditions to provide a path to ground and to induce an equal voltage in the accompanying line for maintaining line-to-line and line-to-ground balance is well known. For example, U.S. Pat. No. 3,496,418, granted Feb. 17, 1970 to Gordon Y. R. Allen, discloses a mutual drainage reactor apparatus which is connected across the two lines of a communication circuit through standard carbon block protectors.
However, these carbon block protectors have very limited current-carrying capacity and will short to ground if large or prolonged currents are passed through them. In normal telephone work it is common practice to use additional devices to protect carbon blocks or gas-filled protectors from such overcurrents. For example, the operating coil of a grounding relay is often connected in series with a protector connected between line and ground with its contacts in parallel with the protector so that when the current through the protector exceeds its rating, the relay contacts will close and bypass the current around the protector until the abnormal circuit conditions subside.